Heretofore, there have been known various types of position detection devices which employ a coil (inductance element) as a detection element. Many of such position detection devices include an AC signal source dedicated to energization of the coil, so that the coil is AC-energized by an AC signal generated from the AC signal source being applied to the coil. Conventionally-known inductive position detection devices are disclosed in Patent Literatures 1 and 2 listed below, for example. Such a conventionally-known inductive position detection device comprises a primary coil and a secondary coil so that the primary coil is energized by an AC signal to induce a secondary output signal in the secondary coil according to the energization, so that inductance of the secondary coil is varied with a relative position, to the coil, of a magnetism-responsive member (e.g., iron, copper, etc) which is displaced in accordance with a position of a detection object to thereby generate the output signal responsive to the position. In this case, an oscillation circuit for oscillating the AC signal for energizing the primary coil is provided separately from the coils.
Also known are proximity sensors which could eliminate the need for a dedicated energizing AC signal source by use of the principle of the LC oscillation circuit, i.e. by incorporating a coil, functioning as a detection element, in a self-oscillation circuit as an inductance element (see, for example, Patent Literature 3). Such self-oscillation type proximity sensors are advantageous in that they can be significantly reduced in size because the need for providing a dedicated energizing AC signal source can be eliminated. However, in a case where an amount of inductance variation in the self-oscillation circuit is small, it is difficult for the sensor to accomplish accurate detection. For example, in a case where a self-oscillation type position detection device comprises a magnetism-responsive member made of iron or the like as a displacement member, conventionally it is difficult to realize the microminiaturization of the device as well as the downsizing of the device.